A few months ago someone introduced MicroAcquire to me as "Failed Micro Startup Marketplace." True to his words most businesses listed there are either <3 years old SaaS in a new niche with low revenue and no profitability or older businesses that could not grow and might not sustain without an exit. Some of these might be a good add-on to an existing business, and that's where they get a good valuation. Flippa is usually a risky site where anyone can list anything for small fees and usually, most of those businesses have either fake numbers or hard to verify. There are a few that might be perfect to build your skills, but don't expect much material value out of those.

Not quite these sites but in the days of niche sites/ marketplaces I knew a few people that purchased eg shopify apps or amazon storefronts and found that to be a more manageable thing to do on the side while working full time